The present invention relates to an apparatus for controlling ultrafiltration in a dialysis system for artificial kidney and a method for controlling ultrafiltration using the apparatus. More particularly, the present invention relates to an apparatus for controlling ultrafiltration adapted to perform an exact and automatic control of the amount of water withdrawn from the patient's blood in a dialyzer and a method for the control.
The method for determining an ultrafiltration rate (hereinafter referred to as UFR) which means the amount of liquid passed per unit of time through a semipermeable membrane from the blood side to the dialyzing liquid side in a dialyzer and controlling a transmembrane pressure (hereinafter referred to as TMP) which means a pressure difference between the blood side pressure of the membrane and the dialyzing liquid side pressure of the membrane, by means of an ultrafiltration controller, may be roughly classified into a positive pressure method and a negative pressure method.
The positive pressure method is such that the pressure difference between the blood side and the dialyzing liquid side, i.e. TMP, is controlled at a desired level by rendering the pressure on the blood side positive. Thus, a blood pump, for example, a roller pump, is provided on the arterial side of the blood circuit to drive the arterial blood. For example, a system based on this principle is described in Japanese Patent Unexamined Published Application (Kokai Tokkyo Koho) No. 59-95053.
The negative pressure method is such that the desired TMP is established by rendering the pressure on the dialyzing liquid side negative. Thus, a negative pressure pump is provided on a discharge line for discharging the used dialyzing liquid from the dialyzer. Apparatuses embodying this latter principle are described in Japanese Patent Unexamined Published Application Nos. 57-66761, 56-84606 and 55-49117, for instance.
While the ultrafiltration controller according to the present invention belongs to the category of negative pressure type, this category of apparatuses may take such constructions as the one employing a flow meter as a means for measuring the amount of ultrafiltrate (see Japanese Patent Unexamined Published Application No. 55-49117), the one employing a volumetric device such as a measuring cup or a metering pump and computing the average value of UFR (see Japanese Patent Unexamined Published Application Nos. 56-84606 and 57-175218), and the one in which a controller designed such that the amount of dialyzing liquid fed to the dialyzer is equalized with the amount of filtrate to be discharged from the dialyzer is used and the amount of ultrafiltrate is computed as a difference between the amount of the filtrate emerging from the dialyzer and the amount of the dialyzing liquid fed to the dialyzer (see Japanese Patent Unexamined Published Application No. 57-66761). Herein, the term "filtrate" is intended to mean the dialyzing liquid discharged from the dialyzer which contains the ultrafiltrate.
As to the method of determining TMP, in view of the difficulty in measuring directly the average pressure on the blood side as well as on the dialyzing liquid side, the TMP computation formula: TMP=P.sub.B -P.sub.D +.DELTA.P, wherein P.sub.B is a pressure on blood side and P.sub.D is a pressure on dialyzing liquid side, with a correction factor .DELTA.P has been proposed to permit a computation of a more exact TMP value (see Japanese Patent Unexamined Published Application No. 59-75057).
However, the conventional ultrafiltration control system of the positive pressure type is disadvantageous in that an exact pressure control cannot be accomplished since the blood level in a blood chamber fluctuates when controlling the blood side pressure.
In the conventional system of the negative pressure type wherein a flow meter is used as a means of measuring UFR, an expensive flow meter or flow chamber must be employed and, in addition, the accuracy of determination tends to drop since metabolic wastes and other foreign matter deposit on these devices. In the system wherein a measuring cup is employed as a means of measuring the amount of ultrafiltrate, the cup can be emptied only by feeding air with a roller pump or the like, so that there are problems not only in the aspect of maintenance but also in terms of cost. Moreover, when UFR is to be measured at a low TMP value and the patient's venous blood pressure is high, the dialyzing liquid side pressure must be rendered positive to ensure the low TMP value in some cases, but when the filtrate is discharged from the measuring cup by exposing the circuit to the atmosphere and the dialyzing liquid side pressure is made positive, the remaining filtrate may flow back into the measuring cup due to compression of air in the ultrafiltration measuring line. Moreover, the filtrate may enter into the measuring cup due to vibrations, etc. prior to measuring.
The system wherein a metering pump is used for measurement of UFR has the disadvantage that in prolonged use the tubing undergoes plastic deformation which results in decrease in the accuracy of determination. Further, the system provided with a means of determining the amount of ultrafiltrate as a difference between the amount of filtrate and the amount of dialyzing liquid fed has the disadvantage that, because many solenoid valves are required, deposits of metabolic wastes and other foreign matter to the valves lead to a decrease in accuracy and sufficient deaeration must be provided so as to prevent attachment of tiny air bubbles.
In regard to the method of controlling ultrafiltration, too, the difference in .DELTA.P between the case in which a dialyzing liquid is flowing through the dialyzer and the case in which the dialyzing liquid is not flowing and the fact that the value of .DELTA.P varies with different patients have not been fully taken into consideration and because of the consequent inaccuracy of .DELTA.P, substantial errors have been inevitable.
It is a primary object of the present invention to resolve the above-mentioned problems and provide an inexpensive ultrafiltration controller device capable of providing an exact and automatic control of the amount of liquid withdrawn from the patient's blood.
Another object of the invention is to provide an ultrafiltration controlling method capable of providing an accurate and automatic control of the amount of liquid withdrawn from the patient's blood.
These and other objects of the present invention will become apparent from the description hereinafter.